Although Garry's Mod is usually considered to be a full game, it has no game objective and players can use the game's set of tools for any purpose. Garry's Mod allows players to manipulate "props" – various objects that players can place in-game. Props can be selected from any installed Source engine game or from a community created collection. The game features two "guns" – Physics Gun and Tool Gun for manipulating objects. The Physics Gun allows objects to be picked up, adjusted and frozen in place. The Tool Gun is a multi-purpose tool for performing various tasks, such as combining props, attaching them via ropes, and creating controllable winches and wheels. The Tool Gun is also used to control add-ons created by the community.
Another popular Garry's Mod concept is ragdoll posing. The player can create a ragdoll model from a Source game and pose it using a variety of tools.[6] This is a popular tool for making fan-made videos.
Garry's Mod takes advantage of the Source engine's modified version of the Havok Physics Engine,[7] which allows players to build contraptions that follow the laws of physics.

I have it so i might be able to describe it a little better. You have a map and some stuff and you build stuff. You know how skyrim is a sandbox rpg? Well Gmod (Garrys mod) is a straight open sandbox. You want to make 10 NPCs fight eachother? You can do it. You want to build elaborate structures? You can. Gmod's only limit is you're imagination. Plus it's cheap as fuck and it's made by god AKA Valve.

Aslo IMPORTANT Despite being a standalone game, Garry's Mod requires the user to own at least one game on Steam based on the Source engine, such as Half-Life 2, Counter-Strike: Source, Portal, or Team Fortress 2.

Yoou have tools and can use props(items)/ragdolls to create many things: funny scenes, cars, constructions and much more. There are alot of different modes to play , like roleplay, sandbox or stranded. There is Wiremod, it adds computer constructions and other things. With it you can build Holograms, timed bombs, special contructions or everything else you could imagine.